Many liquids are or can be sprayed electrostatically. Some particular examples are pesticides or other agricultural chemicals, paints, lacquers, adhesives, release agents, and so on. One feature of electrostatic spraying which is usually of advantage, is that because the droplets in the spray carry an electrostatic charge, they tend to deposit more reliably on the target. Less of the liquid being sprayed is wasted.
Electrostatic spraying apparatus is known in which liquid is drawn out preponderantly by electrostatic forces into ligaments which break up into electrically charged droplets. In order for that to happen the electric field strength must be sufficiently high. In order to reduce the voltage required to produce a sufficient field strength, it is known to supply the liquid to a sharp edge, the shape of which intensifies the electric field, and from which the liquid sprays.
In the prior art, when a plurality of ligaments is produced from one edge, at any given flow rate the number of ligaments which form depends on the field strength at the edge. Increasing the field strength increases the number of ligaments. Increasing the number of ligaments at the same overall flow rate, has the effect that each ligament is finer so that the droplets it breaks up into, are smaller. Thus increasing the electric field strength at the edge, reduces the droplet size.
Unfortunately, the field strength at the edge depends on the distance between the edge and the earth boundary of the electric field. The effective earth boundary is the target. Thus the droplet size depends very significantly on the distance from the target. When the distance from the target increases, the droplet size increases. A technique for producing an intense electric field which overcomes this problem, is described in British Pat. No. 1569707. Here the electric field is defined between a spraying edge and an earthed electrode, usually referred to as a field adjusting electrode (FAE), adjacent the edge. Because the electrode is so much nearer the edge than the target, the electric field strength at the edge is largely independent of the distance from the target. Thus, provided other parameters such as flow rate and voltage are controlled, the droplet size is very largely independent of the distance from the target.
An interesting feature of this apparatus is that the electrode can be positioned so that virtually none of the droplets produced deposit on the electrode.
Further, since the field strength can be accurately defined, it is possible to balance the voltage and the position of the electrode so that in use the field strength is insufficient to produce a corona discharge. That enables an apparatus to be powered by torch batteries and thus to be portable, which had not been possible previously since corona discharge had previously led to a rather heavy current requirement.
A significant part of the cost of the apparatus is the cost of the high voltage generator. One possibility for reducing the cost of the generator, would be to allow greater tolerance in its output voltage by finding another mechanism for controlling droplet size.
Another possibility for reducing the cost of the generator is to reduce the current flow still further. It is now speculated that the nearness of the electrode to the edge may cause a significant leakage via the materials of the apparatus, in use, even though that is much smaller than had previously been produced by corona.
A means of controlling droplet size is therefore sought which does not require a closely regulated voltage output and which does not introduce as short a potential leakage path.